In humans and other animals, normal body function involves many energy consuming activities. A great deal of energy is consumed by bodily movement such as walking and running Activities that occur even at rest, such the heart beat, breathing, blood circulation, maintenance of blood pressure, maintenance of homeostasis, and maintenance of muscle tone, also consume energy. Most of the mechanical energy used for these functions is not reused, but is instead returned to the environment as thermal energy.
Mechanical energy can be converted into electrical energy by moving a coil of conductive material (e.g., a coil of wire) in a magnetic field. Typically, the source of mechanical energy may be a reciprocating or turbine steam engine, water falling through a turbine or waterwheel, an internal combustion engine, a wind turbine, a hand crank, or compressed air.